A traditional acoustic echo canceller (AEC) is often used in phones to at least partially cancel acoustic echo caused by coupling from a loudspeaker of the phone to a microphone of the phone. The acoustic signal from the loudspeaker can travel along many acoustic paths to the microphone, such as through the air via reflections off nearby objects, and through the body of the phone itself. While the acoustic path through the phone body is typically fixed, the environment around the phone may change at any time, and so the AEC is typically adaptive to continuously change its model of the total acoustic environment.
In some phones, especially low-cost phones, the acoustic isolation between the microphone and the loudspeaker in the phone body tends to be fairly poor. This creates a fixed acoustic path within the phone body from the loudspeaker to the microphone. Furthermore, the attenuation presented by this internal acoustic path tends to be much less than the corresponding attenuation associated with the external acoustic paths. As a result, the internal acoustic path may dominate the overall acoustic coupling characteristics between the loudspeaker and the microphone of the phone, as compared with the external acoustic environment.